The present invention relates to an exhaust purification system and a method for controlling an exhaust purification system. In particular, the invention relates to an exhaust purification system and a method for controlling an exhaust purification system capable of clearing a blockage of a reducing agent injection valve caused by solidification of an aqueous urea solution.
Hitherto, an exhaust gas of an internal combustion engine mounted on a vehicle includes a nitrogen oxide (hereinafter, referred to as a “NOx”) or a particulate material (hereinafter, referred to as a “PM”).
Among these, there is known a urea SCR system as a device which reduces the NOx so as to purify the exhaust gas. The urea SCR system includes a reducing agent supply device which supplies an aqueous urea solution as a reducing agent pumped up from a storage tank by a pressure-feeding pump from a reducing agent injection valve into an exhaust pipe and an SCR catalyst which is a kind of exhaust purification catalyst capable of adsorbing ammonia. In such a urea SCR system, ammonia which is produced by the decomposition of the aqueous urea solution is adsorbed to the SCR catalyst and the NOx in the exhaust gas reacts with the ammonia in the SCR catalyst, thereby purifying the exhaust gas.
Meanwhile, as a device which collects the PM to purify the exhaust gas, there is known a diesel particulate filter (hereinafter, referred to as a “DPF”). The DPF is disposed in the exhaust pipe of the internal combustion engine and collects the PM in the exhaust gas when the exhaust gas passes through the DPF. In the exhaust purification system including the DPF, in order to prevent the blockage of the DPF, forced regeneration control is performed at an appropriate timing in which the temperature of the DPF is increased to be about 500 to 600° C. so as to forcedly burn the PM deposited on the DPF.
In recent years, an exhaust purification system having both the DPF and the SCR catalyst increases in number as the purification standard of the exhaust gas becomes strict.
Incidentally, the urea SCR system is generally configured to collect the aqueous urea solution remaining in the reducing agent supply path when the internal combustion engine is stopped (for example, see JP 2009-215891 A). Accordingly, it is possible to prevent a problem in which the aqueous urea solution is frozen while remaining in the reducing agent supply path and blocks the reducing agent supply path.